Conventional filters used in a high-frequency range include a filter using a ¼-wavelength or ½-wavelength resonator including micro-strip or coplanar line, which is a planar filter expected to have smaller dimensions.
Waveguide tube filters which can be expected to have a lower loss include a dielectric waveguide tube filter, which is smaller in dimensions compared to a rectangular waveguide tube. In the dielectric waveguide tube filter described in Patent Publication JP-A-11-284409, for example, and shown in FIGS. 11A and 11B. the waveguide tube is configured by forming conductor layers 2a and 2c (FIG. 11B ) on the top and bottom surfaces of a dielectric substrate 1, the top conductor layer 2a and the bottom conductor layer 2c are connected together through via-hole arrays 3a, which are formed so that a spacing 1p (FIG 11 A) along the signal transfer direction is equal to or less than ½ of the in-tube wavelength. In addition, via-holes 3b constituting the inductive windows are formed in the waveguide tube thus configured so that the spacings (11, 12, 13 and 14) are equal to or less than ½ of the in-tube wavelength, thereby realizing a filter.
However, in the planar filter, since the electromagnetic wave is concentrated in a narrow area, the loss thereof increases due to the conductor loss or dielectric loss. In addition, since the electromagnetic wave expands outside the dielectric substrate constituting the planar filter, there is a problem in that the filter characteristic is changed due to the influence by a package when it is mounted on the package.
Further, as for the dielectric waveguide tube filter described in JP-A-11-284409, if a filter having a steep out-of-band suppression characteristic is to be achieved therefrom, the filter will have a larger number of stages and thus larger dimensions. Thus, there also arises a problem in that designed characteristics cannot be achieved due to limited manufacturing accuracy.